Uptake of organic anions by the liver has kinetic characteristics of carrier-mediated transport. In previous studies an organic anion binding protein (OABP) from a sinusoidal-enriched rat liver cell plasma membrane subfraction was identified and purified. Monospecific antibodies raised to OABP have permitted its quantitation by ELISA and its detection by immunoblotting. Studies of liver distribution by immunocytochemistry reveal that OABP is limited to the sinusoidal and lateral surface membrane of the hepatocyte. There is no reactivity seen in canaliculi, within cells or in different cell types. Several model systems in which organic anion transport and OABP concentration are reduced have been described (e.g. development, regeneration), and studies of BSP transport have been extended to short-term cultured rat hepatocytes. These cells are easily manipulated and permit studies of liver cell transport without the hemodynamic and hormonal alterations which may complicate studies performed in intact animals. The long-term goal is to understand the mechanism of organic anion transport and its relationship to specific proteins in normal liver and in various acquired and inheritable disorders. The specific aims are to: (1) Define saturation kinetics, temperature dependence, and specificity of BSP transport in short-term cultured hepatocytes from normal rats and from rats in which BSP uptake is reduced (e.g. regeneration, nafenopin treatment) and to study the potential function of OABP in these cells by the preparation and use of specific radiolabeled affinity probes; (2) Prepare cDNA probes for OABP from phage liver libraries screened with anti-OABP or oligonucleotide probes synthesized according to the sequence of CNBr peptide fragments of OABP. These probes will permit studies of regulation of OABP expression and also permit sequencing of OABP to determine identity or areas of homology with known proteins; and (3) Study cellular and subcellular distribution of OABP by immunocytochemistry in liver, cultured hepatocytes, and other organs that contain OABP and transport organic anions (heart, brain, kidney, and GI tract).